Two-dimensional neural processing of auditory information in mammalian auditory cortex

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We studied 2-dimensional activities of auditory cortex in guinea pigs and rats using a 4x 4 channel microelectrode-array technique and an optical recording method with a 12 x 12 channel photodiode array. The aim of this study was to examine how the auditory cortex processes auditory information in a two-dimensional way along the cortex. The animals were anesthetized with Nembutal (30 mg/kg) and neuroleptanalgesia (droperidole 16.7 mg/kg/h and pentazocine 5 mg/kg/h). For optical recording, the auditory cortex was stained with a voltage sensitive dye RH795 (styryl dye, Molecular Probes, Inc.). Color-coded spatiotemporal response patterns were displayd on a monitor screen. Sound stimuli used were pure tone bursts, frequency modulated (FM) sounds and vocalizations of the animals. They were presented to the ear contralateral to the recording side.Tonotopical representation of the auditory cortex in guinea pigs and rats was confirmed by the present study using these 2-dimensional recording s … More ystems. We found newly that this tonotopical representation through the functional organization of the cortex is only transient and it varies with time. The neural activity in the guinea pig auditory cortex usually initiates ventrally and it propagates toward dorsally along the cortical surface. The moving direction is parallel to the isofrequency band. For FM sound stimulation, the neural activity propagates obliquely across the isofrequency bands. When a direction of FM sweep is reversed, the direction of propagation is reversed in the frequency axis, but not in the dorsocaudal axis. By applying bicuculline, a GABA_A antagonist, to the auditory cortex, we found strong GABAergic inhibition between isofrequency bands. However, cross-correlation analysis between action potentials of neuron pairs recorded by the microelectrode-array revealed that neurons located in about 0.5 mm separation along the cortical surface rarely showed neural correlations. Vocalization-specific responses were not found in the present study. These results indicate that parameters of sounds are represented in a 2-dimensional way along the auditory cortex, and that the auditory information was transformed into places within the auditory cortex. Less